Vast quantities of cellulosic materials occur universally as surplus and waste residues of agricultural operations. In particular, they occur in the form of straws of cereal grains, for example, wheat, barley, corn rice, oats and rye (sometimes termed stovers, for example, cornstover). Basically, these materials contain cellulose, usually in combination with significant amounts of hemicellulose and smaller amounts of lignin. Because of their carbohydrate content, these materials represent potentially valuable renewable resources for animal feed production.
Various known processes have been proposed or used to convert various cellulosic materials into products which are purported to be suitable, as substitutes for soymeal and similar protein-rich substances, for animal feed protein rations through the utilization of the cellulose as the carbon source for the fermentation of microorganisms to protein-enriched biomass. Cellulosic materials are generally resistant to direct solid phase utilization in such fermentations and require pretreatment to render the cellulose in a form which is utilizable by the microorganism as a carbon source.
In one such prior art process, yeasts are cultivated on liquid monomeric sugar solutions which are produced by chemical hydrolysis of the hemicellulose and/or cellulose components of the cellulosic material. A typical example of this type of process is that described in Han et al U.S. Pat. No. 3,937,845, wherein there is described the hydrolysis of straw using sulphuric acid and the subsequent utilization of the hydrolyzed sugars as a carbon source for the growth of yeasts, together with Trichoderma viride, to form animal feed.
In another prior art process, the cellulose material is initially pretreated to depolymerize or degrade lignin in the cellulosic material before cultivation of cellulase-elaborating bacteria of the genus Cellulomonas on the pretreated material. The pretreatment involves the use of aqueous sodium hydroxide solution of concentration from 2 to 50% by weight at temperatures from 25.degree. C. to 100.degree. C. and treatment time of 15 to 90 minutes. Such a process is described in Callihan et al U.S. Pat. No. 3,761,355 and Srinivasan et al U.S. Pat. No. 4,062,727.
Prior art procedures have, in general, suffered from a number of drawbacks which are considered highly undesirable. Processing costs tend to be high because conversion rates are low, and/or extreme pretreatment conditions of temperature and/or pressure are required to be utilized for pretreatment of the cellulosic material, and/or the pretreatment liquor must be discarded. In some cases, the product is not suitable as animal feed protein ration, owing to a too low protein content and a too poor protein quality. In addition, some products are unsuitable owing to toxicity and indigestibility problems.
It has previously been disclosed by the inventor herein and his coworkers, in an article entitled "SCP Production by Chaetomium cellulolyticum, a New Thermotolerant Cellulolytic Fungus", published in Biotechnology and Bioengineering, vol. XIX, pages 527-538 (1977), that Chaetomium cellulolyticum (ATCC No. 323319), a (then) newly-isolated cellulolytic fungus, showed 50 to 100% faster growth rate and over 80% more final biomass-protein formation than Trichoderma viride, a well-known high cellulase-producing cellulolytic organism, when certain cellulose materials were used as the sole carbon source in the fermentation media, and further the amino acid composition of the product compares more favorably with alfalfa and soya meal than Trichoderma viride.
The cellulosic materials included sawdust, a forestry residue material, and this material was subjected to partial delignification by treatment with 1% NaOH for 1 hour at boiling followed by boiling for 1 hour in peracetic acid. This severe pretreatment procedure was chosen since it was commonly used to treat cellulosic materials to prepare the same for use of the carbon sources in the fermentation of other microorganisms, such as Cellulomonas species. In addition, the pretreatment liquor was discarded prior to fermentation of the solids.